Disk drives are digital data storage devices which allow host computers to store and retrieve large amounts of data in a fast and efficient manner. A typical disk drive includes a plurality of magnetic recording disks which are mounted to a rotatable hub of a spindle motor and rotated at a high speed. An array of read/write heads is disposed adjacent to data storage surfaces of the disks to transfer data between the disks and a host computer. The heads can be radially positioned over the disks by a rotary actuator and a closed loop servo system, and can fly in close proximity to the surfaces of the disks upon air bearings. The heads each typically contain a separate read element and write element.
Higher data storage density on the disks may be obtained by reading and writing data on narrower tracks on the disks, and by maintaining narrower flying height gaps between the heads and the data storage surfaces. The flying height of a head can vary in response to air pressure changes in the disk drive, and in response to head temperature variations which can affect the distance that the tip of the head protrudes therefrom (i.e., pole-tip protrusion). Maintaining the head fly height within an acceptable range is becoming increasingly more difficult as that range is reduced to obtain higher data storage densities. Operation outside the acceptable range may result in an unacceptable read/write bit error rate and/or undesirable contact between a head and a data storage surface and potential loss of data and/or damage to the data storage surface.
Some prior art techniques for measuring flying height have included various read signal harmonic ratio measurement techniques and optical techniques, such as those disclosed in U.S. Pat. No. 4,777,544 and U.S. Pat. No. 7,038,875, both of which are incorporated by reference herein as if set forth in their entireties.